The technique of digitally encoding a video signal and particularly a composite video signal, is well known. Sampling pulses are generated in synchronism with a color video burst signal. The sampling pulses have a repetition rate that is a multiple of the burst signal frequency. Each sample of the color video signal is encoded, or digitized, such as by pulse code modulation (PCM). Digitally encoded video signals are used in time base error correction devices, noise suppression devices, the addition of various special video effects and the like. Digitally encoded video signals are particularly advantageous for recording and reproduction and also for special types of transmission.
In many video transmission systems, color or chrominance information is represented by a particular phase of the chrominance subcarrier signal that is amplitude modulated with color information. Since the phase of the color subcarrier signal is used to represent color information, it is important that, when digitally encoding the color video signal, the phase of the sampling pulses be accurately controlled. Undesired phase shifts, such as may be due to temperature drift, aging of the electrical components, and the like, may result in a phase error in the sampling pulse relative to the chrominance subcarrier signal which has the effect of distorting or interfering with the overall chrominance effect of the video picture which ultimately is reproduced from the digitally encoded video signal.
To identify the aforementioned phase shifts between the sampling pulses and the chrominance subcarrier signal, the instantaneous phase angle of the burst signal at the time of sampling is determined. If the phase angle of the burst signal differs from a desired phase angle, the phase of the sampling pulses may be adjusted accordingly. If the phase angle of the burst signal is different than an expected phase angle, the phase difference between the modulated chrominance information and the signal will also be in error, thereby causing distortion of the color within an output video signal.
A composite color video signal includes horizontal synchronizing signals, a burst signal superimposed onto the pedestal level at the back porch of the horizontal synchronizing signal and a video information signal. The video information signal comprises a chrominance subcarrier having different phases amplitude-modulated with chrominance information. The composite color video signal includes both luminance and chrominance information.
Separator circuits are utilized to separate the horizontal synchronizing signal and the burst signal from the incoming video signal. The burst signal has a burst signal frequency equal to 3.58 MHz, which is the frequency of the chrominance subcarrier f.sub.SC.
A schematic block diagram of a digital encoder of the prior art, as taught by Tatami in U.S. Pat. No. 4,404,583, is illustrated in FIG. 1. In this digital encoder the incoming video signal is sampled at the sampling rate 4f.sub.SC. The sampling pulses which are supplied to the sample-and-hold circuit 2 from the clock generator circuit 5 are phase-synchronized with the incoming burst signal and, generally, each sampling tracks phase shifts present within the burst signal. However, the actual phase of the sampling pulses relative to, for example, the phase of the chrominance subcarrier signal, may drift from the desired phase-lock relationship. This drift may be due to temperature changes in temperature-sensitive circuit components, changes in the operation of such components due to age, and the like. As a result of this phase shift in the sampling pulses, errors, such as phase errors, may be introduced into the sampled video signal, thereby introducing errors into the digitally encoded color video signal. If left uncorrected, the color of the video picture reproduced from the encoded video signal, having the phase errors, may be erroneous.
In order to correct the above-mentioned phase errors, a phase detecting circuit 7 is included within the digital encoding circuit. This sampling phase detector 7 is coupled to the output of the A/D converter 3 and is adapted to produce a control signal which is a function of the phase error, if any, within the signal. If the phase of the sampling pulses generated by the clock generator circuit 5 and supplied to the sample-and-hold circuit 2 drifts, the sampling phase detector 7 is adapted to detect this drift and supply a compensating control signal to the phase shift circuit 6. This compensating control signal controls the phase shift circuit to adjust the phase of the sampling pulses so as to restore the desired, predetermined phase relationship thereof.
The sampling phase detector 7 is adapted to receive the digitally encoded samples of the burst sample, as output by the A/D converter 3, and to derive therefrom an indication, or representation, of the actual phase angle at which the burst signal is sampled. When the sampling pulses exhibit the proper phase relationship with respect to the burst signal, the burst signal will be sampled at a predetermined phase angle thereof. However, if the determined sampled phase of the burst signal differs from this predetermined phase angle, the sampling phase detector 7 outputs a control signal used to control the phase shift circuit 6 in order to shift the phase of the sampling pulses so as to bring them into coincidence with the predetermined burst phase angle.
The clock generator circuit 5 includes a phase comparator 10, a low pass filter 11, a voltage controlled oscillator (VCO) 12 and a frequency divider 13. The VCO 12 is adapted to generate a local oscillating signal whose frequency is equal to the sampling frequency 4f.sub.SC. The output of the VCO 12 is frequency-divided by the frequency divider 13. The dividing ratio of the frequency divider 13 is equal to four such that the frequency divider produces a frequency-divided local oscillating signal whose frequency is equal to the burst signal frequency f.sub.SC. This frequency-divided local oscillating signal is supplied through the phase shift circuit 6 as an input to the phase comparator 10. The other input of the phase comparator 10 is coupled to the separator circuit 4 and is adapted to receive the burst signal within the composite video signal. Any phase difference between the phase-shifted, frequency-divided local oscillating signal and the received burst signal is detected by the phase comparator 10 and output as a phase error. This phase error signal from the phase comparator 10 is then filtered by the low pass filter 11 and supplied as a control signal to the VCO 12. The phase of the sampling pulses generated by the VCO 12 is adjusted in response to the phase error signal from the phase comparator 10 so as to reduce the phase error signal to a null value.
The use of a clock generating circuit having a VCO 12 is well known in the art. However, the use of a VCO in a phase control circuit has a tendency to introduce noise and other unwanted effects into the digitally encoded composite video signal. What is needed is an automatic phase control circuit which does not include a voltage controlled oscillator, thereby reducing the susceptibility of noise on the phase control circuit.